PHARMACOKINETIC PROFILES OF ORAL PHENYLBUTAZONE, MELOXICAM, AND FIROCOXIB IN SOUTHERN BLACK RHINOCEROS (DICEROS BICORNIS MINOR).

The pharmacokinetic profile of selected NSAIDs in southern black rhinoceros (Diceros bicornis minor) were studied. Phenylbutazone (PBZ), meloxicam (MEL), and firocoxib (FIR) were administered orally to five captive, black rhinoceros, and blood was collected at predetermined time points for NSAID quantification and noncompartmental pharmacokinetic (PK) analysis. Phenylbutazone 4.0 mg/kg PO q12h for three doses, MEL 0.3 mg/kg PO q24h administered twice, and a single oral dose of FIR 0.1 mg/kg, were tested with a minimum washout time of 2 wk. PBZ reached a median (range) peak concentration (Cmax) of 9.42 (2.74-11.5) g/ml at a mean (range) time (Tmax) of 6.00 (4.00 to >12.00) h, and the median (range) elimination half-life (T1/2) was 6.07 (3.95-6.49) h. Phenylbutazone pharmacokinetic parameters for black rhinoceros in this study were similar to domestic horses. Meloxicam reached a median (range) Cmax of 0.576 (0.357-0.655) µg/ml at a median (range) time (Tmax) of 6.00 (4.00-12.00) h; the median (range) T1/2 of MEL was 14.0 (12.4-17.9) h. These results demonstrate that once-daily administration of MEL at 0.3 mg/kg resulted in a serum concentration of greater than 0.200 µg/ml from 2 to 24 h in four animals, which is within the analgesic range (0.200-0.400 µg/ml) for this drug in other species postulated by other studies. A single dose of firocoxib (0.1 mg/kg) reached a median (range) peak concentration (Cmax) of 15.7 (9.65-17.3) ng/ml at a median (range) Tmax of 4.00 (4.00-6.00) h. The median (range) elimination T1/2 of FIR was 4.96 (4.47-6.51) h, which is faster than in the horse. The data suggest that extrapolation from equine FIR dosage recommendations is inappropriate for black rhinoceros.

Pharmacokinetics of methocarbamol and phenylbutazone in exercised Thoroughbred horses.

Methocarbamol (MCBL) is commonly used in performance horses for the treatment of skeletal muscle disorders. Current regulatory recommendations for show horses and racehorses are based on a single oral dose of 5 g, although doses in excess of this are often administered. The goal of the current study was to characterize the disposition of MCBL following higher dose administration and administration in combination with another commonly used drug in performance horses, phenylbutazone (PBZ). Exercised Thoroughbred horses were administered various doses of MCBL as a sole agent and MCBL in combination with PBZ. Blood samples were collected at various times, concentrations of MCBL and PBZ measured using LC-MS/MS and pharmacokinetic parameters calculated using compartmental analysis. Following administration of 15 g of MCBL, either as part of a single- or multiple-dose regimen, a number of horses exceeded the Association of Racing Commissioners International and the United States Equestrian Federation's recommended regulatory threshold at the recommended withdrawal time. There was not a significant difference between horses that received only MCBL and those that received MCBL and PBZ. Results of the current study support an extended withdrawal guideline when doses in excess of 5 g are administered.

Comparative pharmacokinetics of phenylbutazone in healthy young-adult and geriatric horses.

OBJECTIVE: To evaluate the effects of aging on phenylbutazone (PBZ) disposition in older horses (≥ 25 years old) compared to young adults (4 to 10 years old) by characterizing the pharmacokinetic profile of PBZ and its active metabolite, oxyphenbutazone (OPBZ), following a 2.2-mg/kg dose, IV. We hypothesized that the disposition of PBZ will be affected by age. ANIMALS: 16 healthy horses (8 young adults aged 4 to 10 years and 8 geriatric horses ≥ 25 years old). METHODS: Horses were administered a single 2.2-mg/kg PBZ dose, IV. Plasma samples were collected at designated time points and frozen at -80 °C until assayed using liquid chromatography-tandem mass spectrometry. Pharmacokinetic analyses were performed using Phoenix WinNonlin, version 8.0 (Certara). Both clinical and pharmacokinetic data were compared between age groups using independent samples t tests, with P < .05 considered significant. RESULTS: Baseline characteristics did not differ between groups, with the exception of age, weight, and plasma total solids. Plasma concentrations of PBZ were best described by a two-compartment model. The maximum plasma concentration of OPBZ was reached at 5 hours for both age groups, and the metabolite-to-parent-drug area-under-the-curve ratios were approximately 20% for both groups. None of the pharmacokinetic parameters of PBZ or its metabolite, OPBZ, differed significantly between age groups. CLINICAL RELEVANCE: The hypothesis was rejected as there was no significant difference in PBZ disposition in young-adult horses compared to geriatric horses. Our data do not support the need for dose adjustments of PBZ in clinically healthy geriatric horses.

Effect of an endurance-like exercise on the disposition and detection time of phenylbutazone and dexamethasone in the horse: application to medication control.

REASONS FOR PERFORMING STUDY: Equine antidoping rules were established to prevent a horse's performance being altered after the administration of prohibited substances, including approved drugs used for legitimate treatment. Veterinarians have to advise owners or trainers on appropriate withholding times to guarantee that their horses may safely compete after drug administration. In order to propose tailored withdrawal times, several horse organisations released detection time (DT) values, for the main veterinary drugs used in horses. One of the possible limits to the information provided by published DTs in horses is the fact that they are determined from classic pharmacokinetic studies performed at rest under laboratory conditions. In field conditions, training and exercise programmes may have an influence on drug elimination. METHODS: Dexamethasone (DMX) and phenylbutazone (PBZ) have been quantified in plasma and urine after solid phase extraction. The kinetic disposition of DXM (8 microg/kg) and PBZ (8 mg/kg) administered by i.v. route in 8 horses, was investigated in rest conditions and during a standardised 3 h test exercise according to a cross-over design. OBJECTIVES: The aim of the present study was to compare the kinetic disposition of 2 test drugs, DMX and PBZ in rest vs. exercising conditions. RESULTS: It was shown in 8 horses that a sustained 3 h of mild exercise slightly decreased the plasma clearance of both drugs (about 25% for DXM and 37% for PBZ) and this is mainly explained by the significant decrease of the corresponding hepatic clearance. In addition, as the volume of distribution was correlatively decreased, the plasma terminal half-life, which is a hybrid parameter of plasma clearance and volume of distribution, remains unchanged overall. CONCLUSION AND POTENTIAL RELEVANCE: Establishing DTs or withdrawal times (WTs) are relevant as plasma and urine half-lives, but not clearance, are the main determinants of DT length. Veterinarians may realistically decide upon a WT for a legitimate drug based on the corresponding DT obtained under resting conditions providing this drug has a low hepatic extraction ratio and a safety margin is added to allow for all possible sources of variability.

Genetic control of drug levels in man: antipyrine.

Antipyrine was administered to identical or monozygotic twins and to fraternal or dizygotic twins. Individuals with identical genotypes (monozygotic twins) exhibited significantly less variability in antipyrine halflife than did genetically different individuals (dizygotic twins). Therefore variations in antipyrine metabolism appear to be determined genetically rather than environmentally. In the 36 twins tested, antipyrine half-lives varied between 5.1 and 16.7 hours. No significant correlation occurred between half-lives for phenylbutazone and antipyrine in the 28 twins who received both drugs.

Genetic control of drug levels in man: phenylbutazone.

Phenylbutazone was administered to seven pairs of identical (monozygotic) twins and to seven pairs of fraternal (dizygotic) twins. Individual half-lives ranged from 1.2 to 7.3 days. Subjects with identical genotypes (monozygotic twins) exhibited very similar phenylbutazone half-lives; significantly greater differences in half-life occurred in dizygotic twins. The previously established large variations among individuals in phenylbutazone metabolism appear to be genetically, rather than environmentally, determined.

Screening, quantification, and confirmation of phenylbutazone and oxyphenbutazone in equine plasma by liquid chromatography-tandem mass spectrometry.

A sensitive liquid chromatographic-tandem mass spectrometric method was developed and validated for screening, quantification, and confirmation of phenylbutazone and oxyphenbutazone in equine plasma. Analytes were recovered from plasma by liquid-liquid extraction followed by separation in a reversed-phase column and identification by mass spectrometry with selected reaction monitoring in negative electrospray ionization mode. Extraction recovery for both analytes was >80%. Limits of detection, quantification, and confirmation for both analytes were 0.01 microg/mL (S/N>or= 3), 0.05 microg/mL, and 0.05 microg/mL, respectively. The assay with d9-labeled phenylbutazone as internal standard (IS) was linear over a range of 0.05-20 microg/mL (r2>0.995). Intra- and interday precision in terms of coefficient of variation was less than 15%. Intra- and interday accuracy (bias%) was within 80-120%. Hemolysis of red blood cells decreased analyte signal intensity but did not affect quantification results because an isotope-labeled IS was used. Analytes were stable in plasma for 24 h at room temperature, 9 days at 4 degrees C, and 45 days at -20 degrees C and -70 degrees C. The method was successfully used in screening, quantification, and confirmation of phenylbutazone in post-competition plasma samples obtained from racehorses. The method is simple, rapid, and reliably reproducible.

Nonsteroidal anti-inflammatory agents and musculoskeletal injuries in Thoroughbred racehorses in Kentucky.

Injuries sustained by horses during racing have been considered as an unavoidable part of horse racing. Many factors may be associated with the musculoskeletal injuries of Thoroughbred race horses. This study surveyed the amounts of nonsteroidal anti-inflammatory agents (NSAIDs) in injured horse's biological system (plasma) at Kentucky racetracks from January 1, 1995 through December 31, 1996. During that period, there were 84 catastrophic cases (euthanized horses) and 126 noncatastrophic cases. Plasma concentrations of NSAIDs were determined by High Performance Liquid Chromatography in injured and control horses. The possible role of anti-inflammatory agents in musculoskeletal injuries of Thoroughbred race horses was investigated by comparing the apparent concentrations of NSAIDs in injured horses to concentrations in control horses. The plasma concentrations of phenylbutazone and flunixin were higher in injured horses than in control horses. Most injured and control horses did not have a detectable level of naproxen in their plasma samples. Further studies must be carried out to determine whether horses with higher plasma concentrations of NSAIDs have an altered risk of musculoskeletal injuries compared with other horses.

Phenylbutazone blood and urine concentrations, pharmacokinetics, and effects on biomarkers of inflammation in horses following intravenous and oral administration of clinical doses.

Phenylbutazone (PBZ) is a potent mon-steroidal anti-inflammatory drug used commonly in performance horses. The objectives of the current study were to describe blood and urine concentrations and the pharmacokinetics of PBZ and its metabolites following intravenous (IV) and oral administration and to describe the duration of pharmacodynamic effect. To that end, 17 horses received an IV administration and 18 horses an oral administration of 2 g of PBZ. Blood and urine samples were collected prior to and for up to 96 hours post drug administration. Whole blood samples were collected at various time points and challenged with lipopolysaccharide or calcium ionophore to induce ex vivo synthesis of eicosanoids. Concentrations of PBZ and eicosanoids were measured using liquid chromatography-tandem mass spectrometry (LC-MS/MS) and non-compartmental pharmacokinetic analysis performed on concentration data from IV and oral administration. Serum concentrations of PBZ and its metabolites were below the limit of quantitation at 96 hours post administration. The volume of distribution at steady state, systemic clearance, and terminal half-life was 0.194 ± 0.019 L/kg, 23.9 ± 4.48 mL/h/kg, and 10.9 ± 5.32 hours, respectively. The terminal half-life following oral administration was 13.4 ± 3.01 (paste) and 15.1 ± 3.96 hours (tablets). Stimulation of PBZ treated whole blood with lipopolysaccharide and calcium ionophore resulted in an inhibition of TXB2 , PGE2 , LTB4 and 15-HETE production for a prolonged period of time post drug administration. The results of this study suggest that PBZ has a prolonged anti-inflammatory following IV or oral administration of 2 g to horses.

Genetic control of dicumarol levels in man.

The mean half-life of dicumarol in the plasma of seven sets of identical and seven sets of fraternal twins after a single oral dose of 4 mg/kg was 43.6+/-SD 17.9 hr. Half-lives ranged from 7 to 74 hr in these 28 normal adults not receiving other drugs for 2 wk preceding dicumarol administration. Large differences among unrelated individuals in dicumarol half-life disappeared almost completely in identical twins, but persisted to some extent in most sets of fraternal twins. These results indicate that marked differences among subjects in dicumarol half-life are under genetic rather than environmental control. Reproducibility of values for dicumarol half-life was demonstrated. A direct relationship between the dose and the half-life of dicumarol occurred in unrelated volunteers administered progressively larger doses at 10-day intervals. Dose dependence of the half-life of a drug results in increased variability of half-life and hence in greater risks of toxicity on long-term therapy. Risks of toxicity on the one hand and of failure to anticoagulate adequately on the other can be reduced by determining dicumarol half-life before starting long-term therapy. Half-lives for dicumarol and phenylbutzone tended to be correlated in the 28 twins, but no correlation occurred between dicumarol and antipyrine half-lives.

Warfarin. Stereochemical aspects of its metabolism and the interaction with phenylbutazone.

An examination of the metabolic fate of the R and the S isomers of warfarin revealed that the two isomers were metabolized by different routes. R warfarin was oxidized to 6-hydroxywarfarin and was reduced to the (R,S) warfarin alcohol. In contrast, S warfarin was oxidized to 7-hydroxywarfarin and was reduced to the (S,S) warfarin alcohol. S warfarin was also oxidized to 6-hydroxywarfarin. These observations suggested that interactions between warfarin and other drugs might be manifest stereo-specifically, i.e., have a different effect on the isomers of warfarin, so a series of experiments were conducted with each isomer of warfarin, before and after phenylbutazone. The plasma clearance of S warfarin was slowed from 3.1 to 1.1% per h in one subject and from 2.3 to 1.6% per h in another. In contrast, the clearance of R warfarin was increased from 1.5 to 3.0% per h and from 0.9 to 1.6% per h in two subjects after phenylbutazone. The rate of clearance of racemic warfarin was unaffected by phenylbutazone; the depression of the rate of clearance of the S isomer masked the stimulation of the clearance of the R isomer. Since S warfarin is five times more potent an anticoagulant than R warfarin, it is concluded that inhibition of the metabolism of S warfarin provides one mechanism for the augmented anticoagulation which follows phenylbutazone.

Induction of a deficiency of steroid delta 4-5 alpha-reductase activity in liver by a porphyrinogenic drug.

The hepatic enzymes that catalyze drug oxidations and the reductive metabolism of steroid hormones to 5alpha-derivatives are localized in membranes of the endoplasmic reticulum. Phenobarbital, which exacerbates acute intermittent porphyria in man, induces drug-oxidizing enzymes in liver. Additionally, patients in whome the primary gene defect (uroporphyrinogen-I-synthetase deficiency) of acute intermittent porphyria has become clinically expressed have low levels of hepatic steroid delta4-5alpha-reductase activity. This 5alpha-reductase deficiency in acute intermittent porphyria leads to the disproportionate generation of 5beta-steroid metabolites from precursor hormones; such steroid metabolites have significant porphyria-inducing action experimentally. In this study the effects of phenobarbital on drug oxidation and steroid 5alpha-reduction in man were examined to determine if this drug could produce changes in steroid 5alpha-reductase activity which mimicked those seen in patients with acute intermittent porphyria. Metabolic studies with [14C]-testosterone and 11beta-[3H]hydroxyandrostenedione were carried out in five normal volunteers. In all five subjects phenobarbital administration (2 mg/kg/per day for 21 days) enhanced plasma removal of the test drugs antipyrine and phenylbutazone as expected; but in four subjects phenobarbital also substantially depressed 5alpha-metabolite formation from [14C]testosterone and resulted in a pattern of hormone biotransformation characterized by a high ratio of 5beta/5alpha-metabolite formation. Studies with 11beta-[3H]hydroxy-androstenedione in three subjects confirmed that phenobarbital produced this high 5beta/5alpha ratio of steroid metabolism by depressing 5alpha-reductase activity for steroid hormones in liver. The high ratio of 5beta/5alpha-metabolites formed in normals after drug treatment mimicks the high 5beta/5alpha-steroid metabolite ratio formed from endogenous hormones in acute intermittent porphyria. The proximate mechanism by which phenobarbital induces reciprocal changes in activities of the microsomal enzymes which catalyze drug oxidations and steroid 5alpha-reductions is not known. This action of phenobarbital raises the possibility, however, that certain drugs which provoke exacerbations of human porphyria may do so, in part, by producing deleterious shifts in the patterns of endogenous steroid hormone metabolism.

Direct-injection screening for acidic drugs in plasma and neutral drugs in equine urine by differential-gradient LC-LC coupled MS/MS.

Direct-injection LC-LC hybrid tandem MS methods have been developed for undertaking broad-based screening for acidic drugs in protein-precipitated plasma and neutral doping agents in equine urine. In both analyses, analytes present in the matrix were trapped using a HLB extraction column before being refocused and separated on a Chromolith RP-18e monolithic analytical column using a controlled differential gradient generated by proportional dilution of the first column's eluent with water. Each method has been optimised by the adoption of a mobile phase and gradient that was tailored to enhance ionisation in the MS source while maintaining good chromatographic behaviour for the majority of the target drugs. The analytical column eluent was fed into the heated nebulizer (HN) part of the Duospray interface attached to a 4000 QTRAP mass spectrometer. Information dependent acquisition (IDA) with dynamic background subtraction (DBS) was configured to trigger a sensitive enhanced product ion (EPI) scan when a multiple reaction monitoring (MRM) survey scan signal exceeded the defined criteria. Ninety-one percent of acidic drugs in protein-precipitated plasma and 80% of the neutral compounds in equine urine were detected when spiked at 10 ng/ml.

Investigation of the role of environmental contamination in the occurrence of residues of the veterinary drug phenylbutazone in cattle.

Phenylbutazone is a non-steroidal anti-inflammatory drug licensed for use in horses to treat musculoskeletal disorders. It is not permitted in the European Union for use in animals destined for the food chain. Official statistics provided by the European Food Safety Authority (EFSA) show that 0.18% of bovines tested in the European Union between 2008 and 2014 for non-steroidal anti-inflammatory drugs were non-compliant, with phenylbutazone representing over 28% of these. Anecdotal evidence suggests animals that have not been treated with the drug may have produced non-compliant samples, possibly through some form of contamination. In this study, ultra-high-performance liquid chromatography coupled with mass-spectrometric detection was applied to bovine plasma samples to determine if detectable residues (CCα = 0.28 ng ml-1) may occur in untreated animals as a result of environmental contamination through normal farming practice. The study demonstrates that waste from animals treated with phenylbutazone, and spread on an area of pasture, can contaminate untreated bovines grazing the pasture many weeks later. It was determined that this contamination, which can persist over a significant period, may be due to the ingestion of as little as 30 µg phenylbutazone by a 500 kg bullock.

Genetic control of interindividual variations in the inducibility of aryl hydrocarbon hydroxylase in cultured human lymphocytes.

Interindividual and intraindividual variations in aryl hydrocarbon hydroxylase (AHH) induction by 3-methylcholanthrene were studied in cultured lymphocytes from normal adult volunteers. Using eight pairs of monozygotic and eight pairs of dizygotic twins, we examined to what extent these variations are controlled by heritable factors and whether AHH inducibility correlations in an individual with the plasma half-lives of three drugs. Substantial overestimation of the induction ratio (fold inducibility) may occur if the nonlinearity of the assay standard curve is not considered. Fold inducibility remains relatively constant for an individual, but large intraindividual variations occur in absolute "control" and "induced" AHH activities. Fetal calf serum may contain inducers of AHH activity that vary with the particular lot of serum, thereby rendering the apparent induction ratio an imprecise indicator of genetic susceptibility to induction by 3-methylcholanthrene. The index of heritability for AHH fold inducibility in twins studied with different lots of fetal calf serum (0.80) or with a single lot of fetal calf serum (0.77) suggests nonetheless that genetic rather than environmental factors are mainly responsible for interindividual variations in AHH inducibility by 3-methylcholanthrene in human lymphocytes. In these twins a significant but poor correlation (r=-0.551; 0.03 less than p less than 0.05) occurs between AHH inducibility in culture and the plasma antipyring half-life, but not between AHH inducibility and phenylbutazone or bishdroxycoumarin half-lives.

Comparison of the effect of Sporobolus virginicus and Rhodes (Chloris gayana) hay diets on the absorption pattern of phenylbutazone in the camel (Camelus dromedarius).

The effect of feeding Sporobolus and Rhodes hay on phenylbutazone (4 g) relative absorption was examined in six camels using a two-period, two-sequence, two-treatment crossover design. Serum concentration of the drug was measured by high performance liquid chromatography. The measured values (means+/-SD) for Rhodes and Sporobolus hay, respectively, were Cmax 35.59+/-22.36 and 36.55+/-18.99 microg/mL, Tmax 26+/-2.53 and 26.3+/-1.97 h and AUC0-72 h 1552+/-872.6 and 1621+/-903.6 microg h/mL. Broad plateau concentrations of phenylbutazone in serum were observed between 12 and 36 h. There was no significant difference in any parameter between the two feeding regimens. Multiple peaks in serum concentration-time curve were observed, regardless of the type of grass available to and the animals prior to drug administration. It was concluded that the phasic absorption of phenylbutazone was a particular feature of hay feeding in camels, and the Sporobolus hay can be fed to camels without any effect on the rate and extent of phenylbutazone absorption compared to Rhodes grass hay.

Intraindividual variation in drug disposition.

Large interindividual differences occur in the in vivo metabolism of drugs due to genetic and environmental factors. Our studies show that intraindividual variabilities in rates of metabolism are relatively low for antipyrine and phenylbutazone, which are drugs that are primarily metabolized by the liver and have low hepatic extractions; whereas in the case of phenacetin, a drug that undergoes extensive metabolism in the gastrointestinal tract or during its first pass through the liver, or both, intraindividual variations in plasma half-lifes and areas under the plasma concentration-time curves are of much greater magnitude. In our studies, no effort was made to control the lifestyles of our subjects. The variations in rates of drug metabolism did not result from assay procedures, since there was little variation in measured concentrations when the drugs were added to plasma and assayed on multiple occasions. Intraindividual variation occurring in subjects given the drug on 5 different occasions may be due to changes in the external environment or changes in internal physiologic parameters or both. Our studies confirm the usefulness of antipyrine as a test drug in studying drug metabolism in man and also demonstrate that the antipyrine test may be able to detect those subjects whose environments are perturbed by unidentified factors.

Effect of oral contraceptives on plasma clearance.

The effects of chronic steroid contraceptive therapy on drug clearance from plasma were studied by using plasma antipyrine, phenylbutazone, and cholecalciferol half-lives in women. After 3 mo of oral steroid therapy (Norinyl, 2 mg; norethindrone + mestranol), the antipyrine half-life was increased in 3 of 6 subjects, phenylbutazone half-life was not consistently altered, and vitamin D3 half-life was increased in 3 of 4 patients. After 1 to 7 yr of oral steroid theraphy, the antipyrine half-life was longer while taking the contraceptive than when the contraceptive treatment was discontinued in 4 of 6 subjects, whereas that of phenylbutazone was not consistently altered.

Mechanisms of inhibition of tolbutamide metabolism: phenylbutazone, oxyphenbutazone, sulfaphenazole.

Tolbutamide half-life was increased by chronic administration of sulfaphenazole (9.5 hr to 28.6 hr, n = 2), phenylbutazone (7.9 hr to 23.1 hr, n = 8), and oxyphenbutazone (8.1 hr to 30.2 hr, n = 2). The rate of elimination of tolbutamide was decreased within 1 to 2 hr of a single dose of sulfaphenazole and the tolbutamide half-life was increased from 9.2 hr to 25.7 hr (n = 2). In contrast, phenylbutazone and oxyphenbutazone, administered as single oral doses of 800 mg, had no immediate effect on tolbutamide elimination. At times greater than 20 to 30 hr after the single dose of phenylbutazone or oxyphenbutazone the rate of tolbutamide elimination was decreased. It is suggested that phenylbutazone and oxyphenbutazone act by inducing form of cytochrome P-450 with low activity for tolbutamide hydroxylation, whereas sulfaphenazole acts by direct inhibition of the microsomal mixed function oxidase system.

Studies in porphyria. V. Drug oxidation rates in hereditary hepatic porphyria.

The mean plasma half-life (T1/2) of antipyrine was prolonged (21.69 +/- 1.92 hr) in a group of 10 patients with hereditary hepatic porphyria, 8 of whom had acute intermittent porphyria (AIP) confirmed by decreased erythrocyte uroporphyrinogen-1-synthetase (URO-S) activities and 2 of whom had mixed hepatic porphyria, in comparison to the mean of 20 normal control subjects (12.65 +/- 0.86 hr, p less than 0.01). Antipyrine T1/2 was especially prolonged in patients with a history of more severe symptoms, but there was no correlation with the degree of elevation in urinary excretion of the porphyrin precursors delta-aminolevulinic acid (ALA) and porphobilinogen (PBG). In 7 completely latent carriers of the AIP gene defect who had normal urinary ALA and PBG levels, the elimination rates of antipyrine from plasma were entirely normal. Phenylbutazone T1/2s were normal in 10 porphyric patients tested. These results demonstrate that the cytochrome P-450-dependent enzyme system for oxidizing antipyrine, but not that for phenylbutazone, is impaired in some AIP individuals in whom the gene defect for the disorder is clinically expressed and that this impairment may be related to the severity of the disease. The partial decrease in URO-S activity characteristic of AIP does not result in a profound or generalized decrease in hepatic cytochrome P-450 function, however, even when there is sufficient derangement in the hepatic heme biosynthetic pathway to lead to excessive excretion of chemical intermediates in the pathway.